The present invention relates generally to systems for polarization demultiplexing within optical transmission systems, and specifically to systems for polarization demultiplexing a higher-speed multiplexed optical signal into lower-speed polarized tributary signals using lower-speed electro-optics to measure the tributary autocorrelation value for use in optimally demultiplexing the higher-speed signal.
In the field of optics and optical transmission systems, multiplexing different data streams for transmission within a system is common. Typically, two or more lower-speed tributary signals are combined or multiplexed together in time slots to form a higher-speed multiplexed signal. For example, two 10 Gbit/sec (lower-speed) tributary signals may be bit-interleaved or multiplexed in alternating time slots to form a 20 Gbit/sec (higher-speed) multiplexed signal. In this way, a single optical path can support transmission of data from multiple sources to multiple receivers.
One way to increase the transmission capacity of such a system is to use optical time domain multiplexing (OTDM) and optical time domain demultiplexing (OTDD). However, the multiplexed transmission capacity of many optical transmission systems is typically limited by the speed of available electro-optics. Multiplexing lower-speed tributary signals into a higher-speed multiplexed optical signal and then demultiplexing the tributary signals out again usually requires wideband electro-optics capable of running at the higher-speed of the multiplexed signal. As the speed of the multiplexed signal increases, the availability of electro-optics that operate at this increased speed unfortunately diminishes and can be a problem for optical transmission system designers.
Applicant has observed that a problem with most OTDD techniques is that they require gating signals at the same or higher-speed as the multiplexed signal. In the previously mentioned example, a gating signal of at least 20 GHz is usually required to handle demultiplexing two 10 Gbit/sec optical tributary signals from a 20 Gbit/sec multiplexed signal. This may be cost prohibitive or impractical as the speed of the multiplexed signal increases.
Furthermore, many existing demultiplexing methods are polarization sensitive, such as techniques using LiNbO3 modulators or conventional four-wave mixing (FWM) to demultiplex higher-speed signals. In these systems, wideband or high-speed driving signals and high-speed electro-optics are often still required to effectively demultiplex the tributary signals from the multiplexed signal.
Patents and publications have described general polarization multiplexing and demultiplexing of tributary signals within optical transmission systems. For example, in an article authored by F. Heismann, P. B. Hansen, S. K. Korotky, G. Raybon, J. J. Veselka and M. S. Whalen entitled xe2x80x9cAutomatic Polarization Demultiplexer for Polarization-Multiplexed Transmission Systemsxe2x80x9d and published in Proceedings, Vol. 2 of 19th European Conference on Optical Communication, published on Sep. 12, 1993 (hereinafter xe2x80x9cthe Heismann articlexe2x80x9d), the authors describe multiplexing two orthogonally polarized optical signals into a single fiber and then demultiplexing them using an automatic polarization demultiplexer. More particularly, the Heismann article describes using a polarization transformer in combination with a simple polarization splitter within the demultiplexer.
Additionally, in an article authored by M. L. Dennis, I. N. Duling III, and M. F. Arend entitled xe2x80x9cSoliton Loop Mirror Demultiplexer with Polarization-Multiplexed Signal and Controlxe2x80x9d and published in Optical Fiber Communication ""96 Technical Digest Series, Vol. 2 on Feb. 25, 1996 (hereinafter xe2x80x9cthe Dennis articlexe2x80x9d), the authors generally describe a nonlinear optical loop mirror-based demultiplexer using orthogonally polarized signals and control streams while operating in the soliton regime. The Dennis article further states that polarization multiplexing of control and signal ensures a high ON/OFF extinction ratio while allowing single wavelength operation.
In accordance with the invention as embodied and broadly described herein, in one aspect, an apparatus is described for polarization demultiplexing a multiplexed optical signal into optical tributary signals within the context of an optical transmission system. In general, the apparatus includes a polarization beam splitter for separating one of the optical tributary signals from the other optical tributary signals. Upon receiving the multiplexed optical signal, separation into tributaries is based upon a polarization relationship. This relationship is preferably an orthogonal relationship between the tributaries in order to provide low crosstalk on the receiving end. Typically, the polarization beam splitter has a first output providing one optical tributary signal and a second output providing another optical tributary signal.
The apparatus also includes a feedback unit which receives one of the optical tributary signals. The feedback unit is optically coupled to an input of the polarization beam splitter. The feedback unit adjusts a polarization state of the multiplexed optical signal based upon an autocorrelation value of one of the optical tributary signals. The autocorrelation value is measured by the feedback unit. Adjustments are typically made to the polarization state depending upon an extinction ratio calculated using autocorrelation values for the signal.
Additionally, the feedback unit typically includes an autocorrelator and a polarization adjustment device. The autocorrelator has input optically coupled to one of the optical tributary signals and provides the autocorrelation value of that optical tributary signal on a low-speed output. The polarization adjustment device has an optical input for receiving the multiplexed optical signal, an optical output coupled to the input of the polarized splitter, and a control input coupled to the low-speed output of the autocorrelator. In this configuration, the polarization adjustment device can adjust the polarization state of the multiplexed optical signal --.--.
The polarization adjustment device may be operative to maximize the autocorrelation value by adjusting the polarization state of the multiplexed optical signal. The polarization adjustment device may also determine an extinction ratio of one of the optical tributary signals based upon the autocorrelation value and maximize the extinction ratio by adjusting the polarization state of the multiplexed optical signal.
The polarization adjustment device typically includes a processing unit and a polarization controller. The processing unit is coupled to the low-speed output of the autocorrelator and provides a feedback signal on its feedback output based upon the autocorrelation value. The polarization controller has an optical input for receiving the multiplexed optical signal, an optical output coupled to the input of the polarized splitter, and a feedback input coupled to the feedback output of the processing unit. In this configuration, the polarization controller can adjust the polarization state of the multiplexed optical signal based upon the value of the feedback signal.
The processing unit may be operative to maximize the autocorrelation value by altering the feedback signal provided to the polarization controller so that the polarization controller can responsively sweep the polarization state of the multiplexed optical signal over a portion of a Poincare sphere. Furthermore, the processing unit may be further operative to determine an extinction ratio of one of the optical tributary signals based upon the autocorrelation value and to maximize the extinction ratio by altering the feedback signal to sweep the polarization state of the multiplexed optical signal over the portion of the Poincare sphere.
In another aspect, a polarization multiplexed optical transmission system is described. In general, the system includes a polarization multiplexer, a high-speed optical path coupled to an output of the polarized multiplexer, and a polarization demultiplexer coupled to the high-speed optical path. The polarization multiplexer has a first input for receiving a first low-speed tributary signal and a second input for receiving a second low-speed tributary signal. The polarized multiplexer can time division multiplex the first low-speed tributary signal and the second low-speed tributary signal into a higher-speed optical signal in a predetermined polarization relationship, such as an orthogonal relationship, with the first low-speed tributary signal being offset from the second low-speed tributary signal in time by a predefined time period, such as a bit period. In other words, a first pulse in the first low-speed tributary signal and an adjacent pulse in the second low-speed tributary signal are offset in time and offset in polarization. The polarization multiplexer avoids coherent mixing of the first low-speed tributary signal and the second low-speed tributary signal by maintaining the predetermined polarization relationship as the tributary signals are multiplexed. This predetermined polarization relationship is typically orthogonal. The high-speed optical path receives the higher-speed optical signal and provides it to the polarized demultiplexer.
The polarized demultiplexer, which includes a polarized splitter and a feedback unit, receives and demultiplexes the higher-speed optical signal. The polarization beam splitter (known more generally as a polarized splitter) receives the higher-speed optical signal and separates the first low-speed tributary signal from the second low-speed tributary signal within the higher-speed optical signal. The feedback unit is optically coupled to an input of the polarized splitter and can adjust a polarization state of the higher-speed optical signal based upon an amplitude characteristic (e.g., an autocorrelation value) of the first low-speed tributary signal measured by the feedback unit.
Additionally, the feedback unit may include an autocorrelator and a polarization adjustment device. The autocorrelator has an input optically coupled to the first low-speed tributary signal from the polarized splitter. In general, the autocorrelator measures the amplitude characteristic and provides it on a low-speed output based upon an autocorrelation value of the first low-speed tributary signal. The extinction ratio of the autocorrelation trace is dependent on the input state of polarization (SOP). The higher the extinction ratio, the more efficient the tributary polarization demultiplexing is. The polarization adjustment device has an optical input for receiving the higher-speed optical signal, an optical output coupled to an input of the polarized splitter, and a control input coupled to the low-speed output of the autocorrelator. The polarization adjustment device is capable of adjusting the polarization state of the higher-speed optical signal based upon the autocorrelation value.
The polarization adjustment device may also maximize the autocorrelation value or extinction ratio (ER) by adjusting the polarization state of the higher-speed optical signal. In this manner, the system advantageously optimizes how the higher-speed optical signal is demultiplexed without resorting to high-speed electro-optics. Furthermore, the polarization adjustment device typically is able to determine an extinction ratio of the first low-speed tributary signal based upon the autocorrelation value and maximize the extinction ratio by adjusting the polarization state of the higher-speed optical signal.
In yet another aspect, a method is described for polarization demultiplexing at least two optical tributary signals into a higher-speed optical signal. The method begins by receiving the higher-speed optical signal and separating a first optical tributary signal from a second optical tributary signal based upon a predetermined polarization relationship between the tributary signals. Typically, the predetermined polarization relationship is an orthogonal relationship. An autocorrelation value of the first optical tributary signal is determined. Based upon the determined autocorrelation ER value, the polarization state of the higher-speed optical signal is adjusted. Typically, the polarization state of the higher-speed optical signal is adjusted and then another autocorrelation value is determined in a repetitive manner in order to maximize the autocorrelation value in a control loop. More particularly stated, the polarization state of the higher-speed optical signal is adjusted by sweeping the polarization state of the higher-speed optical signal over a portion of a Poincare sphere and repeating the determining step in order to maximize the autocorrelation ER value.
Additionally, an extinction ratio of the first optical tributary signal may be determined over a predefined period of time. Typically, the autocorrelation value is used to help determine the extinction ratio for the tributary signal. Accordingly, the polarization state of the higher-speed optical signal can be adjusted based upon the extinction ratio. Furthermore, the polarization state of the higher-speed optical signal can be adjusted by sweeping the polarization state of the higher-speed optical signal over a portion of a Poincare sphere and repeating the determining step in order to maximize the extinction ratio.
Due to the orthogonality of the polarization multiplexed tributaries, maximizing the autocorrelation extinction ratio of one polarization demultiplexed tributary will maximize the extinction ratio of the other polarization beam splitter port. This means that the control electronics and polarization adjustment are needed only for one tributary.